Efforts to improve the performance of natural mineral oil based lubricants by the synthesis of oligomeric hydrocarbon fluids have been the subject of important research and development in the petroleum industry for at least fifty years and have led to the relatively recent market introduction of a number of superior polyalpha-olefin (PAO) synthetic lubricants, primarily based on the oligomerization of alpha-olefins of 1-alkenes. In terms of lubricant property improvement, the thrust of the industrial research effort on synthetic lubricants has been toward fluids exhibiting useful viscosities over an extended range of temperature, i.e., improved viscosity index, while also showing good lubricity, thermal and oxidative stability and pour point equal to or better than mineral oils. These new synthetic lubricants may exhibit lower friction and hence increase the mechanical efficiency of the equipment in which they are used, for example, mechanical loads such as worm gears, gear sets, and traction drives as well as in engines and they may do so over a wider range of operating conditions than mineral oil lubricants.
Notwithstanding their generally superior properties, PAO lubricants are often formulated with additives to enhance those properties for specific applications. The more commonly used additives include oxidation inhibitors, rust inhibitors, metal passivators, antiwear agents, extreme pressure additives, pour point depressants, detergent-dispersants, viscosity index (VI) improvers, foam inhibitors and the like, as described, for example, in Kirk-Othmer "Encyclopedia of Chemical Technology", 3rd edition, Vol. 14, pp. 477-526, to which reference is made for a description of such additives and their use. Significant improvements in lubricant technology have come from improvements in additives.
Improvements have also come from new base fluid development for inherently better properties. Alkylated aromatics, particularly alkylated naphthalenes, are known to possess useful antiwear properties, thermal and oxidative stability as disclosed in U.S. Pat. Nos. 4,211,665, 4,238,343, 4,604,491 and 4,714,7944, making them suitable for use as heat transfer fluid, and functional fluids. The antiwear properties of alkylnaphthalene lubricating fluids are disclosed in Khimiya i Tekhnologiya Topliv i Masel, No. 8, pp. 28-29, Aug., 1986.
Recently, high VI lubricant compositions (referred to here as HVI-PAO) comprising polyalpha-olefins have been disclosed in U.S. Pat. Nos. 4,827,064 and 4,827,073. The process for making these materials comprises, briefly, oligomerizing a C.sub.6 -C.sub.20 1-alkene feedstock such as 1-decene with a reduced valence state Group VIB metal catalyst, preferably a reduced chromium oxide on a porous silica support, to produce high viscosity, high VI, liquid hydrocarbon oligomers which have a characteristic structure with a branch ratio less than 0.19. The oligomers are also characterized by good flow properties, usually having a pour point below -15.degree. C. Lubricants produced by the process cover the full range of viscosities from low viscosity lubricants such as 5 cS fluids to higher viscosity lubricant additives useful as VI improvers, for instance, oligomers having a viscosity of 1,000 cS or more, as described in U.S. application Ser. No. 07/345,606, to which reference is made for a description of these high viscosity materials and their preparation. These high viscosity oligomers, too, exhibit a remarkably high VI and low pour point even at high viscosity. The as-synthesized HVI-PAO oligomer has olefinic unsaturation associated with the last of the recurring monomer units in the structure and accordingly, the oligomer will usually be subjected to a final hydrogenation treatment in order to reduce residual unsaturation to make a final, fully stable product.